Aromatic polyimides represent a class of high-end polymers. They have inherent good properties, such as wear and friction properties, good electrical properties, radiation resistance, good cryogenic temperature stability and good flame retardant properties. Therefore, aromatic polyimides are used in the electronics industry for flexible cables, as an insulating film on magnet wire and for medical tubing. Polyimide materials are also used in high or low temperature exposed applications as structural parts where the good temperature properties are a prerequisite for the function.
Various types of aromatic carboxylic acid dianhydride monomers and aromatic diamine monomers have been used to obtain various types of aromatic polyimides. Examples of aromatic carboxylic acid dianhydride monomers which have been used include pyromellitic dianhydride, 4,4′-oxydiphthalic anhydride, 2,2-bis-[4-(3,4-dicarboxyphenoxyl)phenyl]-propane dianhydride, 3,3′,4,4′-benzophenonetetracarboxylic acid dianhydride or 3,3′,4,4′-tetracarboxybiphenyl dianhydride. Examples of aromatic diamine monomers which have been used include 4,4′-oxydianiline, 1,4-diaminobenzene, 1,3-diaminobenzene, 1,3-bis-(4-aminophenoxyl)benzene, 1,3-bis-(3-aminophenoxyl)benzene, methylenedianiline or 3,4′-oxydianiline.
Williams and Donahue, (U.S. Pat. No. 3,983,093) have shown that the solvent resistance of polyeytherimides may be improved by using a rigid aromatic carboxylic acid dianhydride, such as pyromellitic dianhydride or 3,3′,4,4′-benzophenonetetracarboxylic acid dianhydride, in addition to an aromatic carboxylic acid ether dianhydride, such as 2,2-bis-[4-(3,4-dicarboxyphenoxyl)phenyl]-propane dianhydride. Further, a related rigid aromatic carboxylic acid dianhydride, i.e. 5,5′-((ethyne-1,2-diylbis(4,1-phenylene))bis(oxy))bis(isobenzofuran-1,3-dione), is described in U.S. Pat. No. 3,956,322.
Furthermore, U.S. Pat. No. 4,973,707 relates to the discovery that polyacetyleneimides, resulting from the intercondensation of an acetylene-di(phthalic anhydride) and an aryl diamine, have high glass transition temperatures, excellent solvent resistance, and improved rigidity compared to polyacetyleneimides of the prior art. The same properties in other polyimides may, according to U.S. Pat. No. 4,973,707, be enhanced by the presence of units derived from 1,2-acetylene di(phthalic anhydride).
According to U.S. Pat. No. 4,973,707, 5,5′-(ethyne-1,2-diyl)bis(isobenzofuran-1,3-dione) may be synthesized in several steps from ethynyltrimethylsilane and 5-bromo-2-methylisoindoline-1,3-dione. In the described synthesis, 5,5′-(ethyne-1,2-diyl)bis(2-methylisoindoline-1,3-dione) is hydrolyzed and subsequently dehydrated to obtain 5,5′-(ethyne-1,2-diyl)bis(isobenzofuran-1,3-dione). Alternatively, 5,5′-(ethyne-1,2-diyl)bis(2-methylisoindoline-1,3-dione) may be obtained in moderate yield by coupling of 2 equivalents of 5-bromo-2-methylisoindoline-1,3-dione with 1 equivalent acetylene. The two proposed synthetic routes provide 5,5′-(ethyne-1,2-diyl)bis(isobenzofuran-1,3-dione) only in low yields (10% and 23%, respectively). Thus, neither of the two proposed routes provide 5,5′-(ethyne-1,2-diyl)bis(isobenzofuran-1,3-dione) in a yield acceptable for industrial scale.
In Chemistry of Materials, 2001, 13, 2472-2475, a three step procedure for obtaining 4,4′-(ethyne-1,2-diyl)bis(isobenzofuran-1,3-dione) from diethyl 3-iodophthalate in moderate yield (52%) is disclosed.
In addition to the use of rigid aromatic carboxylic acid dianhydride, other ways for improving various properties, such as mechanical properties, of polyimides for use in airplanes and aerospace applications are known in the art.
As an example, the processability of polyimides may be improved by introducing cross-linking monomers into the polymer. As the resulting polymer chains may be cross-linked, they may be shorter whilst the mechanical properties are maintained or even improved. Shorter polymer chains have the advantage of being easier to process, as the viscosity of the polymer melt is lower. Examples of such cross-linking technologies include bismaleimides and nadimide-based PMR resins, which undergo cure at temperatures near 250° C. However, such thermoset polyimides will not withstand oxidative degradation on long-term exposure at temperatures above 200° C., as the cross-linking moieties have inferior thermal stability, compared to the oligoimide units.
In attempts to improve the thermal stability, thermoset polyimides containing phenylethynyl-substituted aromatic species as the reactive end-cappers have been developed. U.S. Pat. No. 5,567,800 discloses phenylethynyl terminated imide oligomers (PETIs). Such oligomers may be prepared by firstly preparing amino terminated amic acid oligomers from dianhydride(s) and a slight excess of diamine(s) and subsequently end-cap the resulting amino terminated amic acid oligomers with phenylethynyl phtalic anhydride (PEPA). The amic acid oligomers are subsequently dehydrated to the corresponding imide oligomers. Upon heating the triple bonds will react and cross-link the end-capped polyimid, thereby improving its heat resistance and mechanical strength.
A process for producing aryl ethynyl phthalic acid, e.g. phenylethynyl phtalic anhydride (PEPA), and derivatives thereof (including fluorine-containing compounds), in which an aryl ethynyl phthalic anhydride is formed by subjecting an aryl phthalic acid to ring closing is disclosed in US 2005/215820.
However, in some applications there is a need for further improving the heat resistance and mechanical strength of PETI. Especially, it would be of interest to allow for improving the mechanical strength of PETI further. In curing of ethynyl group modified oligomers and polymers, such as PETI, the curing temperature and yield of cross-linking is to a large extent determined by the mobility of the ethynyl group. A more mobile group will have a lower curing temperature and give rise to higher yield of cross-linking. Hence, ethynyl groups used in the art for cross-linking has typically been positioned at the ends of the oligomers and polymers to be cross-linked, cf. PETI, as the end-groups will have higher mobility compared to other parts of the oligomers and polymers.
The degree of cross-linking, which may be achieved, is inherently linked to the ratio of cross-linking groups and polymer chains. The portion of cross-linking end groups may be increased by decreasing the length of the polymer chains. However, decreasing the length of the polymer chains will lower the heat resistance and especially the mechanical strength. Further, the polymeric properties will be decreased and eventually lost if the length of the polymer chains is decreased.
The present inventors have found (cf. WO 2012/131063), that the degree of cross-linking may be enhanced by combining the use of an phenylethynyl terminated end-capper, such as PEPA (cf. U.S. Pat. No. 5,567,800) or PETA (cf. WO 2011/128431), with use of an acetylene-di(phthalic anhydride), such as 5,5′-(ethyne-1,2-diyl)bis(isobenzofuran-1,3-dione).
However, in order for such a combined concept to find wide spread industrial application, there is need for an alternative synthetic route to provide 5,5′-(ethyne-1,2-diyl)bis(isobenzofuran-1,3-dione) and related (ethyne-1,2-diyl)bis(isobenzofuran-1,3-diones) in high yields and adequate purity. Especially, the obtained 5,5′-(ethyne-1,2-diyl)bis(isobenzofuran-1,3-dione) should preferably have low halogen content, as halogens catalyze degradation of polyimides at high temperatures. Further, halogen may hamper the incorporation of 5,5′-(ethyne-1,2-diyl)bis(isobenzofuran-1,3-dione) into polyimides as the imidization is negatively affected by the presence of halogens. In addition, the isolating effect, being an essential property of polyimide films used in electronics, is negatively affected by the presence of halogens and the risk for treeing is thereby increased.
According to U.S. Pat. No. 5,185,454, the halogen content of di-aryl acetylenes obtained via Sonagashira couplings may be reduced via treatment with water. While 5,5′-(ethyne-1,2-diyl)bis(2-methylisoindoline-1,3-dione) may be washed with water to reduce the halogen content, 5,5′-(ethyne-1,2-diyl)bis(isobenzofuran-1,3-dione) may not, without hydrolyzing the carboxylic moiety, be washed with water.
Thus, there is need within the art for a process for obtaining 5,5′-(ethyne-1,2-diyl)bis(isobenzofuran-1,3-dione) in high yields and adequate purity for incorporation into polyimides.